The gas transport properties of two different polymerized ionic liquids (PILs), a homopolymer poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethanesullonyl)imide) [poly(MEBIm-TFSI)] and a diblock copolymer poly(methyl methacrylate-block-1-[(2-methacryloyloxy)undecyl]-3-butylimidazolium bis(trifluoromethanesulfonyl) imide) [poly(MMA-b-MUBIm-TESI)], were investigated at different temperatures and humidities to determine the effect of water vapor on the separation efficacy of these PlL membranes. In addition, pure water sorption experiments were performed to quantify the water content in the PILs under these different experimental conditions. Under dry conditions, the permeabilities and selectivities of different gaseous penetrants were tested (CO2, CH4, N-2 and He), where both PIL membranes showed suitable features for CO2/CH4 and CO2/N-2 separation: a CO2 permeability of 33 Barrer at 35 degrees C and selectivities up to 17 and 24 for CO2/CH4 and CO2/N-2 separation, respectively. Under humid conditions, the CO2 and CH4 permeabilities increased linearly with increasing water concentration within the PlL membranes. At 90% RH, 80% and 40% enhancements were observed for the hompolymer and block copolymer, respectively, compared to dry conditions. Interestingly, water uptake was observed to be rather limited, never exceeding 2.5 wt% for both PILs under the experimental conditions in this study. Since the permeability enhancement appeared to be similar for both CO2 and CH4, a negligible effect on the gas selectivity was observed under humid conditions, thus leading to an overall improvement in both PIL membrane gas separation performances. (C) 2015 Elsevier B.V. All rights reserved.